David Robson examines the role of lasers in producing low-cost, efficient solar cells that could provide a green replacement for fossil fuels
In March 2007, member governments of the European Union signed a binding agreement to source 20 per cent of their energy from renewable resources by 2020. While biofuels look set to become a popular choice of fuel to help countries achieve these targets, concerns over the environmental impacts of dedicating yet more land to agriculture mean that we will also need to investigate other, older technologies that have yet to realise their initial potential fully.
One such technology is the solar cell. But while, on the surface, it appears to be an ideal source of energy by making using of an (almost) universally available resource – the Sun – most of us have yet to see the benefits in anything more energy consuming than a solar-powered handheld calculator.
This is largely because the conversion of light energy to electrical energy has been inefficient, so very large, unsightly and expensive solar cells are necessary to produce sufficient energy to power larger devices. However, manufacturers are now developing techniques to produce solar cells with higher efficiencies, at a lower cost, meaning they could soon become a viable option to power whole buildings or even cities.
Finlay Colville, the director of marketing for micro material processing at Coherent, believes that ‘the laser is a key enabler’ for these advances. Lasers provide delicate scribing, welding, and cutting that will help to maximise the amount of energy produced in traditional, silicon waferbased solar cells. They are also instrumental for the production of a new, cheaper type of solar cell, built from very thin coatings of materials, which could be applied to flexible surfaces that could range from tents and rucksacks to curved buildings.
Silicon wafer-based solar cells Silicon wafer-based solar cells are by far the most prevalent solar cell technology, currently occupying about 90 per cent of the market. These cells are made from a wafer of silicon, and for this reason many of the manufacturing techniques are similar to the processes used to reduce the size of semiconductor chips computers, many of which use lasers. ‘The whole production can draw from the semiconductor industry,’ says Colville.
‘Everything is about increasing the efficiency and yield.’ These solar cells are typically made from two different types of silicon crystal, which are doped with different substances to form p-type and ntype semiconductors. When these two different crystals touch, they produce a slight electric field that encourages the movement of electrons freed by sunlight to form a current that can then be harnessed.
Typically, the p-type silicon forms the core of the cell, and is completely coated with the n-type silicon. However the front of the cell (which faces the sun) must be isolated from the back of the cell, which connects to the electric circuit, to prevent short circuits that would ruin the production of electricity. To disconnect the front of the cell from the back, lasers are used to scribe a groove around the edge of the cell that cuts through the outer layer of silicon crystal.
This groove needs to be as narrow as possible to avoid wasting expensive semiconductor wafer. It is not possible to scribe lines narrower than the wavelength of the laser, so lasers that produce short-wavelength green and UV light are now used to make these marks as small as possible.
These shorter wavelengths are also less likely than longer wavelengths to produce tiny cracks under the surface of the silicon. ‘Any stress applied to the cell, when it is made into a panel or when it is installed, can cause these cracks to grow that limit the yield of electricity,’ explains Colville. Lasers could also be used to embed the contacts that conduct the generated current to an electric circuit. Previously, the contacts would be placed on the surface of the semiconductor.
This created shadows that blocked the sunlight from a small portion of the surface of the solar cell, wasting some of the potential light energy. However, a new technique currently in its pilot stage embeds the contacts within the semiconductor, reducing this effect. To achieve this, the lasers scribe a tiny trench of just 20-30μm in width that is then electroplated to form the contact.
Sometimes, it is best to place all of the contacts on the back of the silicon cell, to maximise the area exposed to the sun. In this case, tiny vias need to be drilled though the solar cell that connect the front face of the cell to these contacts at the back surface. Lasers are used to drill these holes. Again, shorter wavelengths are preferable to reduce the size of the holes and minimise damage to the silicon wafer.
In addition to these relatively minor adjustments, manufacturers are constantly trying to reduce the thickness of the silicon wafer itself, which should reduce the cost of the devices. ‘If solar power is going to be huge, we need to decrease the wastage of the raw materials,’ says Colville.
These new wafers, which could be just 100μm in thickness, will be very brittle. Lasers are much gentler than mechanical processes and cheaper than photolithography, so it is likely that they will become increasingly popular for the above techniques as manufacturers look to further increase the efficiency of their production.
Thin-film solar cells Thin-film solar cells are made from multiple layers of semiconductor materials, layered in thicknesses of just a few micrometers. It is hoped that, in the future, they could be produced on very large scales,
with a simpler manufacturing process than silicon wafer-based solar cells, which could significantly reduce the costs of production. However, a current drawback is that these cells produce electricity at just half the efficiency of wafer-based solar cells.
Richard Hendel, the international sales manager for solar technology at Rofin, believes that it could be up to 10 years until the two technologies produce comparable results. The solar cells can be manufactured in large plates of up to a square metre in size. A single solar cell of this size would create a very high current, which would produce undesirable heating effects that dissipate valuable electrical energy. To reduce this current while still maintaining a high voltage, the surface is processed to isolate 200 individual cells within the larger plate, which are then connected in series.
Lasers are used to isolate and connect the different cells, by engraving lines in the coatings as soon as they are deposited. This is a fairly standard process for lasers, which are very accurate and don’t waste any of the material. Qswitched lasers are used for this, with good pulseto- pulse stability that allows users to finely control the amount of energy that is applied to the coatings.
‘You need very good beam quality that can be focused to a very small point (around 15μm), and you need to have the right pulse energy and frequency, which could range from 50-100kHz,’ says Hendel. With these qualities a laser can scribe at speeds of greater than 1m/s, providing a very fast and efficient manufacturing process.
‘Lasers are faster and they are able to cut trenches a lot thinner than chemical processes,’ he says. ‘It’s not been proven that any other technology can do this process.’ Despite the obvious advantages of lasers in these processes, it seems that the solar cell industry is moving so fast that the question of which particular laser to use for these processes is unimportant in comparison with other factors that could affect the reliability and efficiency of their products.
‘The whole industry is 10 years away from optimisation, and the choice of laser is very low down in their criteria,’ says Coherent’s Finlay Colville. ‘As a brand-new industry it started using diode pumped solid-state lasers, and there is no push to move to a different technology.’
Aside from the fine adjustments that may be necessary once the solar industry has matured, it seems that lasers have already proved themselves as an essential component in the production of these devices.
Some historians of science believe that, as we run out of fossil fuels, the ‘green revolution’ may be just as radical as the industrial and digital revolutions. If this is the case, and solar cells really do live up to our expectations, we can be sure that lasers will have played a key role in this change.
As further evidence of improvements to lasers used for solar cell production, new work has returned promising results. Dr Tony Hoult, and a team at SPI Lasers’ US applications facility, has shown that a 200W continuous-wave 1070nm fibre laser can cleanly cut the polycrystalline silicon used in solar cell production, with little debris production.
Cutting speeds of up to 360m/s, on 200μm silicon ribbon have already been achieved. Analysis of the cuts shows very smooth surfaces with minimal debris or spatter, and with no appreciable taper.
SPI has now joined with the Cambridge Institute of Manufacturing (CIM) in the UK, and other laser-based research institutes around the world with similar equipment, to conduct more detailed tests and further trials.